Add checkstop gpio to Firestone, Garrison, Witherspoon

Firestone, Garrison, and Witherspoon were missing the checkstop
gpio handling. Add the checkstop handler process and the gpio
pin number per the system schematics.

Change-Id: Id53e2f8888a7c706bf30a95fe56223628a2ba70a
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
3 files changed